Because of manufacturing defects, mounting strains, and environmental (e.g., temperature) changes, many optical systems perform in the field in a manner inferior to the system design and original construction. A notorious example of this is the Hubble Space Telescope.
On a more down to earth level, photolithography equipment, such as that used in the manufacture of semiconductor devices, provides an example of optical equipment which typically performs in a degraded manner in actual usage. Small deviations from the design specification will lead to errors which, although seemingly small, can represent a substantial contribution to the overall error budget allowed for the manufacture of these intricate devices.
More particularly, the "taking" (imaging) lens of semiconductor photolithographic apparatus must exhibit especially high imaging quality. In use, however, the image will tend to be somewhat degraded, often in unpredictable ways. This is often attributable to the shape, or curvature of the taking lens deviating from its design specification and/or original construction. Persistent, endemic problems can be detected by conventional gauging equipment, such as may be used in acceptance testing of the lens or at a later time to verify the continued proper performance of the lens. (See, e.g., U.S. Pat. No. 4,984,886.) The remedy for this class of problems is simply to exchange the lens for one which better matches the design specification, but this can represent an unwelcome expense and, more importantly, a major interruption in the up-time of a system.
When an optical element, such as a lens or mirror, is exhibiting sporadic degradation of imaging quality, in use, the problem can be inconsistent, harder to diagnose and somewhat elusive.